1. Field of the Invention
The present invention pertains to wireless communications, and particularly aspects of wireless communications including power saving operations and frequency selection/change.
2. Related Art and Other Considerations
Power conservation has long been a concern for computers, particularly computers which utilize a battery power source to some degree. Power conservation is an important factor for computers which are connected in wireless manner to a local area network (LAN), including portable or mobile computers or other telecommunications units including the same.
A computer such as a personal computer (PC) typically has various devices which communicate over one or more buses with each other. Often some of these devices can have a power save feature. For a PC which executes an operating system of the Microsoft® Windows® family and forms part of a LAN, the power save feature for these devices has become known as “Wake-On-Lan”. Wake-On-Lan is used to achieve low power consumption, but still permits connectivity for incoming information from the LAN to the PC.
The Microsoft® Wake-On-Lan power saving implementation has power modes both for the devices connected to the PC bus, as well as “system” power modes. In view of the fact that wireless LANs are employed, e.g., to facilitate mobility of the computer or workstation, the “system” in the context of a wireless LAN can be referred to as a “mobile terminal” (MT) or “user equipment” unit (UE). The mobile terminal (MT) includes a device (e.g., a card or circuit board) which is utilized to interface to the wireless network, such card often being referred to as a wireless Network Interface Card (NIC). Thus, the terminology “mobile terminal (MT)” encompasses both the Network Interface Card (NIC) and the computer which accommodates the Network Interface Card (NIC).
In the Microsoft® Wake-On-Lan scheme, a device can have power modes D0, D1, D2, and D3, ranging from mode D0 (which has no power saving at all) to mode D3 (the deepest power save mode). The device modes are denoted herein as Dx, where x is one of 0, 1, 2, or 3. The system has power modes S0, S1, S2, S3, S4, and S5. In mode S0, the power for the system is fully on, whereas mode S4 is a hibernate mode (a very deep sleep mode but in which the system is able to resume operation with a reboot). The S5 mode means that the system is off and that a reboot is the only way to resume system operation. Using similar notation as with the device modes, the system generally modes are denoted as Sx.
With the Wake-On-Lan functionality, an interrogation is performed between the devices connected to a common bus prior to entering a lower power mode, e.g., modes D1, D2, or D3. The least capable mode (i.e., highest power mode) for any of the devices on the bus is selected as the preferred mode for the bus. There are specific requirements for each of the device power saving modes, including power consumption requirements. The higher the number of the mode, the lower is the power consumption.
Wake-On-Lan can work from any Dx mode as long as the system mode is less than S5 and the device is able to follow the preconditions and requirements for that Dx mode (e.g., power consumption requirement), and still be able to detect received frames. When a device detects a wake-up event, the device signals the system accordingly. This signaling is accomplished in different ways depending on which bus the device is placed on. For example, on a CardBus the signaling is accomplished using a line CSTSCHG#.
Two types of standards for wireless LANs are briefly described below. The first type of standard is the HIPERLAN (HIgh PErformance Radio Local Area Network) standard; the second type of standard is the IEEE 802.11. HIPERLAN (HIgh PErformance Radio Local Area Network) is an ETSI (European Telecommunications Standards Institute) standard for wireless LANs, there being existing standards for HIPERLAN Type 1 and proposed standards for HIPERLAN Type 2 (H2). IEEE 802.11 is an IEEE (Institute of Electrical and Electronics Engineers) standard for Wireless LANs.
Concerning first the HIPERLAN standards, H2 is based on a time-division multiple access/time-division duplex protocol. In H2, an in-building component operates at 5 GHz and the second generation of the standard operates at at least 24 Mbits/s sustained, with potentially even higher data rates.
In H2, the mobile terminal (MT) communicates with an Access Point (AP). The LAN may have several Access Points (AP), each of which is essentially analogous to a base station serving a cell. Since H2 is time division duplex (TDD), both the mobile terminal (MT) and the Access Point (AP) use the same frequency. The Access Point (AP) is connected to an operator's intranet. The intranet can be, for example, for a company or industrial complex, office, hospital, airport, or even home or private use). The Access Point (AP) of the cell selects, preferably with respect to its cell autonomously, the best frequency for communicating with a particular mobile terminal (MT). In the best frequency selection, the Access Point (AP) utilizes measurements taken both by the Access Point (AP) itself, as well as measurements taken by the mobile terminal (MT) and forwarded to the Access Point (AP). In this regard, the measurements can be, for example, interference measurements (e.g., signal to noise ratios [SNRs]) for various eligible frequencies for selecting a best available frequency.
A Medium Access Control (MAC) layer is utilized for controlling communications between the Access Point (AP) and an mobile terminal (MT). In particular, the Medium Access Control (MAC) layer for the wireless LAN is a reservation-based MAC layer.
Assuming that a particular mobile terminal (MT) has been authenticated and a connection has been established, various transactions typically occur. Discussion of these transactions is facilitated by an understanding of the MAC frame structure shown in FIG. 11. The frame is typically 2 ms, and a Broadcast Control Channel (BCCH) is always sent in every frame. The BCCH includes a pointer to the beginning of a random access channel (RCH) start, and the number of random access channels included in the frame. The RCH itself is a transport channel of nine byte size that can contain various logical information protocol data units. In addition to RCH, other transport channels such as Broadcast Channel (BCH), FCH, ACH, SCH, LCH, and RCH are utilized, all of which are filled with logical channels. If it were assumed that there is only one sector per cell, then there would be one BCH, one FCH, and one ACH per frame. However, there can be multiple SCH, LCH, and RCH per frame. ACH is a feedback channel that conveys the Access Point (AP) reception status of the RCHs. LCH is used to signal user data and control data. SCH is used for feedback signaling of the user data reception status (ARQ) and also for controlling signaling. RCH can be filled with resource requests, connection setup messages, and control messages. The Access Point (AP) has a scheduling entity which is responsible for filling the frames with the random access channel opportunities, e.g., RCHs.
Concerning the transactions mentioned above, in order to send Uplink (UL) data the mobile terminal (MT) has to monitor a Downlink (DL) Broadcast Control Channel (BCCH), which is transmitted in the Broadcast Channel (BCH). This monitoring of the Downlink (DL) Broadcast Control Channel (BCCH) is for the purpose of ascertaining where the random access channel (RCH) opportunities are located in the frame. The random access channel (RCH) is utilized by the mobile terminal (MT) in order to request UL resources. The random access request is acknowledged by the Access Point (AP), and the Access Point (AP) starts scheduling the UL resources for the mobile terminal (MT) in a time division duplex (TDD) airlink traffic channel, i.e., the reservation based on the requested access begins.
Upon reception of the Downlink (DL) at the Access Point (AP) of data from the fixed network (e.g., the operator's intranet) which is destined to the mobile terminal (MT), the Access Point (AP) can either (1) buffer such data if the mobile terminal (MT) is sleeping (e.g., defer the transmission of such data until after the Access Point (AP) has waken the mobile terminal (MT)); or (2) transmit the data at the next possible occasion. Data is announced by the Access Point (AP) to the mobile terminal (MT) by broadcasting a MAC-ID of the mobile terminal (MT) in the Frame Control Channel (FCCH) that is transmitted in the Frame Channel (FCH) sent after the BCH [see FIG. 11]. The FCCH also contains the exact location where the data is carried in the DL phase.
HIPERLAN Type-2 facilitates battery lifetime conservation using a sleep mode at the mobile terminal (MT), known herein as the H2 sleep mode. In order to enter the H2 sleep mode, the mobile terminal (MT) has to petition the Access Point (AP) explicitly for permission to sleep. When in H2 sleep mode, a mobile terminal (MT) will monitor BCCH only periodically. The periodicity for such monitoring of the BCCH is negotiated between the mobile terminal (MT) and the Access Point (AP).
Currently it is envisioned that H2 shall involve no manual frequency planning, but that the frequency used by each Access Point (AP) is selected by a Dynamic Frequency Selection (DFS) algorithm. The Dynamic Frequency Selection (DFS) algorithm is implemented in the H2 radio access network itself, e.g., at the Access Point (AP), on the basis of received signal strength (RSS) measurements. As mentioned above, the Access Point (AP) utilizes both measurements performed by the Access Point (AP) itself as well as measurements taken by the mobile terminal (MT) in the cell managed by the Access Point (AP). In this regard, upon command from the Access Point (AP), the mobile terminal (MT) can measure both the RSS of the frequency currently being utilized for communication with the Access Point (AP), as well as other frequencies. The particular frequencies which the mobile terminal (MT) is to measure, and in what frame, is assigned by control signaling from the Access Point (AP).
Thus, the performance of the Dynamic Frequency Selection (DFS) algorithm depends upon measurements from the mobile terminal (MT). In HIPERLAN Type 2 as currently envisioned, the Access Point (AP) knows basically whether the mobile terminal (MT) is in an H2 sleep mode or not, but knows very little else about the mobile terminal (MT) power status.
Various problems can arise in H2 operations depending on the periodicity of the measurements required from the mobile terminals (MTs), as is illustrated in two distinct cases described below. A first such case focuses on a problem for the mobile terminal (MT). As is readily understood, it is highly desirable for the mobile terminal (MT) to minimize power consumption during either an active mode or a sleep mode. In the sleep mode, the active/inactive duty cycle is very low, in the range of 1/1000 for sleep duration of ten frames. The duty cycle depends on the periodicity by which the mobile terminal (MT) monitors the BCCH. Depending on the measurement requirement dictated by the Access Point (AP), the signal strength measurements on adjacent frequencies very easily will add up to the magnitude of 1/1000. In other words, if constant measurements of adjacent frequencies are required in the sleep mode, battery consumption will become a problem. Or, since battery consumption is a non-negotiable parameter, the measurements must be infrequent to avoid high battery consumption, but on the other hand, the greater risk is to loose any vital information.
Power consumption must, of course, include both the power consumption in the Network Interface Card (NIC) and also the power consumption in the PC. It is likely, but not necessarily the case, that the PC in sleep mode state D0 consumes much greater power than the Network Interface Card (NIC) in sleep mode state D0. If, then, measurement is required by the Access Point (AP), for a Network Interface Card (NIC) in sleep mode D3, requires a wake up of the PC in order to increase the power state from D3 to D0 (or other), then it is likely that the power consumption in the PC is the critical aspect. Depending on the system state Sx (x=0–5), after the wake up the increase in power consumption can be reduced, e.g., it would be unnecessary to turn on the monitor device, for example. This again leads to the same problem of decreased battery lifetime for the mobile terminal (MT).
The second case involves the Access Point (AP). PC power management will sometimes force the mobile terminal (MT) to enter a lower power mode, e.g., D1, D2, or D3. There are requirements (e.g., power consumption requirements) to enter these different states, the lower the state Dx (x=1–3), the higher is the requirement upon low power consumption. Depending upon the power consumption for each vendor device, there it may be more or less problematic to support the measurement within a low power mode.
As an aside, it is mentioned in passing that the foregoing may lead to a workaround described above to wake up the PC. In this case, however, the assumption is that the Network Interface Card (NIC) defers to do the measurement.
From the point of view of the Dynamic Frequency Selection (DFS) algorithm of an Access Point (AP), the condition is negative since the device may then not be able to perform the measurement, and the Access Point (AP) will not be aware of it other than the fact that measurement reports will not be received. If multiple mobile terminals (MTs) in a cell are unavailable to measure upon request from the Access Point (AP), then the lack of measurement samples may lead to a performance degradation in the cell. For the mobile terminal (MT), the problem will occur that even though the measurement request has been properly received, no action can be taken since it would drain the battery availability and lead to a synchronization loss or similar fatal error.
Decoupled from the Dynamic Frequency Selection (DFS) measurement problem, but tightly coupled to the battery problem, is that the requested sleep periodicity affects the power consumption for the mobile terminal (MT). Shorter sleep cycles increase power consumption but shorten the response time upon mobile terminated transactions.
IEEE 802.11, the second type of standard mentioned above, is based on a carrier sense multiple access collision avoidance (CSMA/CA) medium sharing mechanism. The medium access control (MAC) supports operation under control of an access point as well as between independent mobile terminals. The second generation of the standard operates at 5 GHz and provides bitrates up to 54 Mbps.
The CSMA/CA procedure requires each device (MT and AP) with pending data to transmit to sense the medium to determine if another device is transmitting. If the medium is not determined to be busy, the transmission may proceed. A transmitting device shall ensure that the medium is idle for a predefined duration before attempting to transmit. If the medium is determined to be busy, the device shall defer until the end of the current transmission. After deferring, or prior to attempting to transmit again after a successful transmission, the device shall select a random backoff interval and shall decrement the backoff interval counter while the medium is idle. The receiver that successfully receives and decodes a protocol data unit acknowledges the reception by transmitting an acknowledgement protocol data data unit back to the sender. Both acknowledged and unacknowledged data transmissions are supported, and specific fields in the data unit header determine the type of transmission.
IEEE 802.11 can be set up both as an infrastructure Basic Service Set (BSS), where an AP via the wireless media provides access for one or multiple MT's, e.g. in an office scenario the AP provides access to the wired LAN for the MT's.
IEEE 802.11 can also be set up as an independent BSS, where MTs via the wireless media can set up a communication network in between the MTs. The term “ad hoc” is often used as slang to refer to an independent BSS. An ad hoc network is typically created in a spontaneous manner, e.g. the MT's in a conference room may create an ad hoc network for the duration of a meeting.
IEEE 802.11 facilitates battery lifetime conservation using a sleep mode at the mobile terminal (MT), known herein as the IEEE sleep mode. For an ad-hoc network an MT requesting to enter a low power mode indicates the low power request by setting a ‘Power Management field’ in the frame control field of a MAC protocol data unit frame. The value indicates the mode in which the MT will be after the successful completion of the frame exchange. The receiver of the frame exchange with the ‘Power Management field’ set to sleep can be any other MT in the ad-hoc network.
An MT that successfully completes a frame exchange with the ‘Power Management field’ set to “sleep” can enter a low power mode until next start of a target beacon transmit time (TBTT). In an ad-hoc network at least one MT transmits periodically a beacon with system parameters. The beacon is transmitted once per beacon interval, with a tentative transmission start at the TBTT. ‘Tentative’ is due to the fact that each MT must sense the activity on the wireless media prior to any transmission. For occasions when another MT transmits at the expiration of TBTT, the beacon transmission must await the transmission. From the TBTT start occasion until a predefined certain period, all sleeping MT will monitor any ‘wake-up’ message destined to the MT. At the absence of a wake-up message the MT can enter a low power mode until next TBTT. If a wake-up message is received an MT must respond to the message and revert to active mode.
For MTs under an infrastructure AP, similar (but not exactly the same) procedures exist. The particular differences are that the MT must inform the AP prior to entering low power mode. The AP then transmits a list of all MTs that have pending data at the AP denoted Traffic Indication Map (TIM). TIM is transmitted in the beacon. Since MT's may sleep for longer period than a beacon interval, the AP must indicate the presence of pending data to a MT in multiple beacon transmissions.
Currently no dynamic frequency selection (DFS) exists in IEEE 802.11, but efforts appear underway to include DFS in the IEEE 802.11 standard. Since DFS relies on measurement results, the similar problems as described for H2 above will, upon such inclusion, exist for IEEE 802.11 as well.
In Wireless LAN system of today, mostly Laptop PCs are utilized. When these units are in operation, e.g. word editing, these units drain a substantial amount of power and thus require batteries that can provide an acceptable battery lifetime. With the introduction of Palmtop devices in wireless LAN systems, the power consumption will be much more critical. To a certain extent each manufacturer may utilize techniques that will lower the power consumption. But it is likely that the wireless LAN standards will have to provide means to enable lower power consumption for a subset of the devices. Currently, neither IEEE 802.11 nor H2 has a mechanism to identify which units are units with low power requirement.